Why store data in files? #

Much larger data storage than RAM.
Persist accross different executions of the program.
Work with other programs.

Opening/Closing a file #

#include <stdlib.h>
#include <stdio.h>
/* File pointer to hold reference to our file */
FILE * fPtr;
// Open file in w (write) mode. "data/file1.txt"  is complete path to create file
fPtr = fopen("data/file1.txt", "w");
/* fopen() return NULL if last operation  was unsuccessful */
if(fPtr == NULL)
{
    /* File not created hence exit */
    printf("Unable to create file.\n");
    exit(0);
}
/* Done with this file, close file to release resource */
fclose(fPtr);

Reading from file #

fgetc() – Used to read single character from file.
fgets() – Used to read string from file.
fscanf() – Use this to read formatted input from file.
fread() – Read block of raw bytes from file. Used to read binary files.

Reading #

Open a file using fopen() function and store its reference in a FILE pointer say fPtr.
You must open file in r(read) mode or atleast mode that support read access.
Read content from file using any of these functions fgetc(), fgets(), fscanf() or fread(). Finally, close the file using fclose(fPtr).

Reading from file, char by char #

do {
    /* Read single character from file */
    ch = fgetc(fPtr);
    /* Print character read on console */
    putchar(ch);
} while(ch != EOF); /* Repeat this 
if last read character is not EOF */

Copying a file with source/destination as commandline arguments #

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv[]) {

    if (argc != 3) {
        printf("Invalid arguments\n");
        return 0;
    }

    char ch;

    FILE* s = fopen(argv[1], "r");
    FILE* d = fopen(argv[2], "w");

    /* fopen() return NULL if last operation was unsuccessful */
    if(s == NULL || d == NULL)
    {
        /* Unable to open file hence exit */
        printf("Unable to open file.\n");
        printf("Please check whether file exists and you have read privilege.\n");
        return 0;
    }

    /* File open success message */
    printf("File opened successfully. Reading file contents character by character. \n\n");

    do 
    {
        /* Read single character from file */
        ch = fgetc(s);

        /* Print character read on console */
        putchar(ch);

        fputc(ch, d);

    } while(ch != EOF); /* Repeat this if last read character is not EOF */

    /* Done with this file, close file to release resource */
    fclose(s);
    fclose(d);

    return 0;
}

Home Work #

Write a program which takes a file name as command line argument and prints the number of chars, words, lines and paragraphs in the file.

Reading line by line using `fgets()` #

char * fgets(char * str, int num, FILE * stream);

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFFER_SIZE 1000

int main() {
    /* File pointer to hold reference to our file */
    FILE * fPtr;

    char buffer[BUFFER_SIZE];
    int totalRead = 0;
    int total_chars = 0;

    /* 
     * Open file in r (read) mode. 
     * "data/file2.txt" is complete file path to read
     */
    fPtr = fopen("1.c", "r");

    /* fopen() return NULL if last operation was unsuccessful */
    if(fPtr == NULL)
    {
        /* Unable to open file hence exit */
        printf("Unable to open file.\n");
        printf("Please check whether file exists and you have read privilege.\n");
        return 0;
    }

    /* File open success message */
    printf("File opened successfully. Reading file contents line by line. \n\n");

    /* Repeat this until read line is not NULL */
    while(fgets(buffer, BUFFER_SIZE, fPtr) != NULL) 
    {
        /* Total character read count */
        totalRead = strlen(buffer);
        total_chars += strlen(buffer);

        /* Print line read on cosole*/
        printf("%s", buffer);

    } 

    printf("Total number of chars: %d", total_chars);

    /* Done with this file, close file to release resource */
    fclose(fPtr);

    return 0;
}

C Standard Library #

[https://en.wikibooks.org/wiki/C_Programming/Standard_library_reference]

[https://en.cppreference.com/w/c/io]

Random Number Generation #

Random Number Generation Reference

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
int main(void)
{
    srand(time(NULL)); // use current time as seed for random generator
    int random_variable = rand();
    printf("Random value on [0,%d]: %d\n", RAND_MAX, random_variable);

    int x;
    int count[] = { 0,0,0,0,0,0};
    for (int i=0; i < 6000; i++) {
       x = 1+ rand()%6;
    //    printf("%d ", x); 
       count[x-1]++;
    }
    printf("%d %d %d %d %d %d\n", count[0], count[1], count[2], count[3], count[4], count[5]);
}

Home Work 1 #

Write a program to generate and print:

a uniformly random subset of {1, 2, .., n}. Take n as input.
a uniformly random subset of {1, 2, .., n} of size k. Take n, k as input.
a uniformly random permutation of {1, 2, .., n}. Take n as input.

Date Time Clock #

Date Time Clock Reference

Home Work 2 #

Write a program to print the number of days in the current month.
Write a program, which takes an input a date and prints the month calendar where that date belongs.

For eg: For today date it should print below:

November 3rd, 2023
Sun Mon Tue Wed Thu Fri Sat  
29  30  31  1   2   3*  4  
5   6   7   8   9   10  11  
12  13  14  15  16  17  18  
19  20  21  22  23  24  25  
26  27  28  29  30

Reading/Writing Binary Data to/from a file #

#include <stdio.h>
#include <stdlib.h>

typedef struct Customer {
    char name[100];
    int phone_no;
} Customer;

int main() {
    Customer c = { "Ramu", 90034699 };
    FILE* cus_file = fopen("customer.bin","w");
    fwrite(&c, sizeof(Customer), 1, cus_file);
    fclose(cus_file);

    Customer d ;
    cus_file = fopen("customer.bin","r");
    fread(d, sizeof(Customer), 1, cus_file);
    printf("Customer Read Details: %s, %d", d.name, d.phone_no);
    fclose(cus_file);
    return 0;
}

Reading/Writing Array of structs #

#include <stdio.h>
#include <stdlib.h>

typedef struct Customer {
    char name[100];
    int phone_no;
} Customer;

int main() {
    Customer c[3] = {
        { "Ramu", 90034699 },
        { "Ammu", 900146939 },
        { "Thomas", 769834234 }
    };

    FILE* cus_file = fopen("customer.bin","w");
    fwrite(c, sizeof(Customer), 3, cus_file);
    fclose(cus_file);
    
    Customer d[3] ;
    cus_file = fopen("customer.bin","r");
    fread(&d, sizeof(Customer), 3, cus_file);
    for (int i = 0; i < 3; i++) {
        printf("Customer Read Details: %s, %d", d[i].name, d[i].phone_no);
    }
    fclose(cus_file);
    return 0;
}

Store Reciept Management System #

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>

typedef struct Customer {
    char name[100];
    int phone_no;
} Customer;

typedef enum PayMode {
    Cash,
    Card,
    UPI 
} PayMode;

typedef struct Reciept {
    time_t time;
    float value;
    // Customer *customer; We cannot store this as pointer.
    int customer_index;
    PayMode mode;
} Reciept;

typedef struct Database {
    Customer customers[100];
    Reciept reciepts[1000];
    int customer_count;
    int reciept_count;
} Database;

int add_customer(char *name, int phone_no, Database *db) {
    Customer *c = &(db->customers[db->customer_count++]);
    c->phone_no = phone_no;
    strcpy(c->name, name);
    return db->customer_count-1;
}

Reciept* add_reciept(int value, int c, PayMode mode, Database *db) {
    time_t now = time(NULL);
    Reciept* r = &(db->reciepts[db->reciept_count++]);
    r->customer_index= c;
    r->value = value;
    r->time = now;
    r->mode = mode;
    return r;
}

int find_customer_by_phone_no(int phone, Database *db) {
    Customer *cust = NULL;
    for(int i =0; i< db->customer_count; i++) {
        if(phone == db->customers[i].phone_no) {
            cust = &(db->customers[i]);
            return i;
            break;
        }
    }
    return -1;
}

void print_db(Database* db) {
    printf("Customers_______________\n");
    for (int i = 0; i < db->customer_count; i++) {
        printf("%s\t%d\n", db->customers[i].name, db->customers[i].phone_no);
    }
    printf("Reciepts_______________\n");
    for (int i = 0; i < db->reciept_count; i++) {
        printf("%d\t%f\n", db->reciepts[i].customer_index, db->reciepts[i].value);
    }
}

Write Example #

int main() {
    Database db;
    db.customer_count = db.reciept_count = 0;

    int a = add_customer("Alice",12345,&db);

    add_reciept(1000, a, Cash, &db);
    add_reciept(500, a, UPI, &db);

    int b = add_customer("Bob",5678,&db);

    add_reciept(700, b, Card, &db);
    add_reciept(500, b, UPI, &db);

    print_db(&db);

    FILE* cus_file = fopen("customers.bin","w");

    fwrite(&(db.customer_count), sizeof(int), 1, cus_file);

    fwrite(db.customers, sizeof(Customer), db.customer_count, cus_file);

    fclose(cus_file);

    FILE* res_file = fopen("reciepts.bin","w");

    fwrite(&(db.reciept_count), sizeof(int), 1, res_file);

    fwrite(db.reciepts, sizeof(Reciept), db.reciept_count, res_file);

    fclose(res_file);

    return 0;

}

Read Example #

int main() {
    Database db;
    db.customer_count = db.reciept_count = 0;

    FILE* cus_file = fopen("customers.bin","r");

    fread(&(db.customer_count), sizeof(int), 1, cus_file);

    fread(db.customers, sizeof(Customer), db.customer_count, cus_file);

    fclose(cus_file);

    FILE* res_file = fopen("reciepts.bin","r");

    fread(&(db.reciept_count), sizeof(int), 1, res_file);

    fread(db.reciepts, sizeof(Reciept), db.reciept_count, res_file);

    fclose(cus_file);

    print_db(&db);

    return 0;
}